1. Field of the Invention:
This invention relates to improvements in communication secrecy systems and is particularly useful where the communication is over public communication channels.
Secrecy systems have been used for many years in military communications. Generally these systems are relatively complex and expensive. In recent times it has become desirable to have secrecy systems for law enforcement agencies and fire departments, as well as other similar agencies employing public communication channels.
2. Description of the Prior Art
Secrecy systems that have been produced in the past have employed such techniques as rolling code and static band splitters to distort the voice signals to provide secrecy. Another technique that has been employed is the rearranging of the voice spectrum on a time basis. A common problem with these earlier attempts to provide secrecy in voice communications is that the code is relatively easy to break for recovery of the message. For example, in the band splitting technique it has been found that if one band is recovered, sufficient intelligence is contained therein for recovery of the message. In these earlier systems the system can generally be broken without looking back at the circuitry used in scrambling the signal. Another approach that has been employed is the scrambling of the voice signals by mixing with a carrier frequency and the subsequent masking of the scrambled signal by application of a masking signal. However, in this system the masking signal may be removed by a simple filter in the decoding circuitry and the code broken thereafter.